This quest will start automatically when you go to the Hanged Man after Tranquility and are on the Questions and Answers quest. You'll see Isabela beating up Lucky and his gang. When you talk to her after that, she'll ask you to provide backup for a duel.

When you get there, you'll find that it's an ambush in disguise. After the battle WAIT for Isabela to ask you to check the bodies. There's a bug that will prevent you from finishing the quest if you don't wait for her before looting the bodies.

You'll find the note leading you to Hayder, you'll have to fight your way through some more of his men before you get to him. After a cutscene it will end with his death. Final result, you'll get Isabela to join your party. This quest will also unlock the Isabela's Contact and Finders Keepers quests. You need to complete this quest to get Isabela as a companion, she's only available to start in Act 1.